I have had pretty consistent success with making macarons using the recipe from Bouchon. Whether I let the shells dry or not, age the egg whites or not, the shells always come out around the same texture and very rarely crack.
What I am running into problems with is this:
I currently only have two baking sheets. To bake the macarons, I pipe onto the first cold sheet and stick it in the oven. While they're cooking, I pipe onto the second cold sheet. Both of these batches come out perfect (save for a few cracked shells due to my uneven oven :-P). When I take out the first sheet, I let the macarons cool, move them to a plate, and pipe the next batch. Same with the second sheet. These last two batches, piped onto the hot baking sheets, ALWAYS come out lop-sided. They rise unevenly, and many only get feet on half of the shell.
Is this unevenness due to the fact that I am piping onto a warm baking sheet? Or is it because the batter is sitting out for 20+ minutes before the third and fourth batches are piped? Everything else about these shells is virtually perfect--no hollows, great texture, shiny top.
How do I avoid this in the future?? More baking sheets??